Wetting and spreading of viscous droplets : a critical assessment of fluid and surface properties

• Main Author: James, Emily
• Other Authors: Harbottle, David ; Cayre, Olivier J. ; Hunter, Timothy N.
• Published: University of Leeds 2018
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.749443

The thesis examines the effects of various parameters on droplet spreading dynamics. A range of PDMS oils with viscosities from 9.9 – 12710 mPa·s were selected and spread on silicon wafers, and the spreading of the droplets were recorded on a pendant drop tensiometer. The droplet radius over time was then used to compare the different spreading dynamics. As expected, increasing droplet viscosity slowed droplet spreading dynamics, removing the viscosity by dividing time through by the oil viscosity created a master curve. The master curve showed that viscosity was the dominating force in droplet spreading. Regions of the curve which deviated from a r ~ t^0.1 spreading were those effected by droplet shape and kinetic energy. Additional kinetic energy showed an increase in spreading dynamics, however this was viscous dependant with a larger effect seen on low viscosity oils. Finally, the CH3-terminated PDMS was compared to NH2-terminated PDMS and it was found that the spreading dynamics for the NH2-terminated PDMS was slower. The spreading dynamics of the PDMS oils were also investigated under D2O, where the viscous forces were also shown to affect droplet spreading dynamics but to a less extent compared to in-air observations. An induction time was also associated with droplet spreading. A thin liquid film (TLF) of water had to first drain and rupture before droplet spreading could take place. NH2-terminated PDMS oils were shown to have a larger extent of wetting than the CH3-terminated PDMS oils, but again had slower spreading dynamics. Silanisation of the surfaces (to display a water-in-air contact angle of 0, 30, 65 and 100°) enabled the effects of surface hydrophobicity to be studied, with a more hydrophobic surface exhibiting faster spreading dynamics and shorter induction times. The addition of a negatively charged surfactant SDS (sodium dodecyl sulphate) showed that with increasing concentration there was a decrease in spreading dynamics and an extension of the induction time, due to Marangoni effects. To explore the effect of surface roughness, cellulose coated wafers (RMS ~ 0.6) were also used, both the CH3- and NH2-terminated PDMS oils showed little spreading, highlighting the importance of surface roughness for droplet spreading. The spreading of inviscid water droplets were also investigated. β-Aescin, a saponin, was found to partition at the air-water interface quickly; after 1 minute of ageing it showed strong interfacial elasticity (G’/G’’ ~ 6). The effect of strong interfacial elasticity on droplet spreading dynamics was then studied for the first time. Droplet spreading dynamics of water, 5 wt % ethanol, 0.0015 wt% N-dodecyl β-D-glucopyranoside and xanthan gum were also studied, to ensure the spreading effects were not due to the solvent, surface tension, surfactant behaviour or bulk rheological properties. The four extra solutions were shown to spread according to a time-dependant power law of 0.5 then 0.1 (Tanner’s law) as expected. However, the 0.01 wt % β-aescin droplets were shown to accelerate droplet spreading dynamics, with an initial power law of 1.05, then 0.61, then 0.1. The elastic film rupture promoted droplet spreading and also enhanced the dampening of oscillatory waves which followed after droplet detachment.